As Alliance and a huge fan of world PvP, killing the leaders of the Horde was extremely fun.
As a WC3 fan, killing Thrall, Cairne and especially Sylvanas was a bit of a downer.
In any case, For the Alliance! was a lot easier than I expected to obtain... so I decided to put up a quick recap for those who may still want this Achievement:
Boss respawn time is a couple of hours.
If a different raid killed the leaders earlier in the day, you may need to wait. Cairne took maybe 2hrs, Thrall was back earlier.
You don't need a full 40man raid.
Unless you're doing it at peak play time and you expect a lot of people on the opposing faction to come and fight you, you can do it with less. We did have a full raid, but most were under 80 (our main tank was a 79 pally). Undercity and Silvermoon were deserted (we didn't even manage to kill 5 players in those cities).
Group up OUTSIDE the city to buff.
Put a raid symbol on someone's head, and make everyone follow them in in a tight bunch through the city. Don't stop until you get to the boss.
Use your mammoth.
Party (not raid) members can use someone's Traveller's Tundra Mammoth, and you won't get dazed/knocked off your mount on your way to the room.
AoE all the guards/players down in the room before starting on the boss.
Guards are level 75, so they go down quick. Players are more annoying, so target them first.
Turn off spell effects
If you're playing WoW on an ancient calculator like I am (I had about 5FPS), turn the graphics down before you start. It's not as bad as the BWL suppression room, but getting there.
People on the Battle for the Undercity quest chain will NOT be able to help in Orgrimmar/Undercity, but will still get the Achievement.
If you're on the quest chain, you'll be in a different version of Orgrimmar/Undercity to the rest of your raid. However, you'll still be able to get the Achievement if you stand in the same room.
Fight in his tent, otherwise you may get punted off TB. You'll need more than 1 tank to spam taunts, because you'll lose aggro pretty often. There is some AoE/raid damage (especially when the tank loses aggro), so the more tanks spamming taunt the easier it is.
Thrall (and Vol'jin)
Go through the back entrance. Both Thrall and Vol'jin are pretty easy (unless the Horde at the Orgrimmar AH want some PvP), although dispellers will need to watch for Vol'jin's Hex. Some raid damage from Chain Lightning etc, but not too bad. We killed Thrall first.
Easiest of the 4. There are extra NPCs around (including the guards), so kill them first.
Sylvanas Windrunner (and Varimathras?)
Use the sewers for entry. Sylvanas is probably the hardest out of the 4 because of the guards and Horde battlemasters. She has a vanish-blink type ability which makes her hard to tank, although disarm apparently works well to counteract this.
1 of the NPCs (not sure which one) also has an annoying AoE fear, so you may want to kill the battlemasters first.
RE: Varimathras.... Because I've completed the Battle for the Undercity quest chain, Varimathras wasn't there for me at all. I don't know if he was there for the people that hadn't completed the quest chain or not, but he didn't seem to be an issue.
At the end of it all, you get a Black War Bear mount. In my opinion Alliance have it easy; Horde have to go through either (or both!) Stormwind or Ironforge in full view of the AH.
For the Alliance!